1. Field of the Invention
The present invention relates to a solid-state imaging device with a plurality of photodetectors disposed in a matrix form to correspond to pixels. More particularly, the present invention relates to a solid-state imaging device provided with a function suited for the use as a high-speed camera.
2. Description of Related Art
A high-speed camera is used for detailed analysis of the test results for crash tests of vehicles, drop tests of products and the like. In recent years, a high-speed camera especially is used also for analysis of a phenomenon in the physical chemistry field including for the analysis of a combustion state of an internal-combustion engine, the analysis of a chemical reaction and the like. Furthermore, along with the development of a digital image processing system, a digital high-speed camera employing a solid-state imaging element such as a charge coupled device (CCD) or a MOS image sensor has become mainstream.
A digital high-speed camera employing a solid-state imaging element can eliminate the film developing process after photography that used to be required for a film camera, and allows an image to be confirmed just after the photography. Also, it can facilitate capturing an image into digital image analysis equipment using a personal computer or the like.
Since a digital high-speed camera allows an image to be confirmed just after the photography, if there is an error of photographing, the remedy is easy because the photography can be repeated again on the spot. Also in terms of the reuse and processing of data, it is easy to process or alter image data obtained by a digital high-speed camera using a personal computer.
Such a digital high-speed camera, however, has the following disadvantages. That is, there is a limit to the number of shots that a digital high-speed camera can photograph continuously because of restrictions of a writing speed to a recording medium or the like. In order to photograph a moving image continuously, at least one of the frame rate and the image size for photography should be reduced so as to allow the writing to a recording medium to keep up with the photography. On the other hand, when the photography is to be conducted with a high resolution and with a high speed, the number of frames that the camera can photograph is limited and the photographing time will be extremely short because the photography should be conducted with short time intervals. For instance, in the case where the frame rate is 100,000 frames per second and the number of frames that the camera can photograph is 100 frames, the photographing time will be 1 millisecond.
In many cases a phenomenon to be captured by a high-speed camera is generated and finished in a short time, and therefore such short photographing time intervals will suffice. However, the timing should be adjusted so that the phenomenon as a photographing target can be included within such a short photographing time. Since the adjustment of the timing is often difficult, under the current state-of-the-art the adjustment with a smaller range has to be repeated until the trigger of the phenomenon agrees with the timing of the trigger of photographing start so that the photographing target can be included within a photographing time in several milliseconds.
JP H05-137074 A for example proposes a method of photographing while monitoring a change in image. According to this method, pixels of a display as a whole are divided into several sections, and the timing for ending the photography is determined based on an average change in contrast obtained from output signals of these pixels. More specifically, so-called first-in first-out (FIFO) memory is used so that old data is overwritten with new data during continuous photography, and when a change in image is detected after the generation of a phenomenon to be captured, a predetermined number of frames are captured to complete the photography.
According to the method disclosed in JP H05-137074 A, however, the detection is prone to delay because the trigger is determined according to an average contrast change of a display as a whole. Moreover, according to this method, since the photography has to be repeated even before the generation of a phenomenon to be captured, it is difficult to reduce the power consumption of the device.